We are currently studying in vitro parameters of immunity to SV-40 transformed cells in mice. The purpose of this work is to define what conditions of immunization will preferentially lead to transplantation responses directed against virus specific antigens and which to responses directed against re-expressed, possibly tumor cross-reactive, fetal antigens. BIBLIOGRAPHIC REFERENCES: L. Gooding, Y.-C. Hsu, and M. Edidin: Expression of Teratoma-Associated Antigens on Murine Ova and Early Embryos. Identification of Two Early Differentiation Markers. Develop. Biol. 49:479, 1976. R.D. Stulting, R.F. Todd, and L.R. Gooding: Susceptibility of Anti-H2-Capped Target Cells to Humoral and T Lymphocyte-Mediated Lysis. Transplantation 21: 71, 1976.